If heavy-metal-ion-containing wastewater arising from processes such as plating, metal surface treatment, battery manufacture, printed circuit board manufacture, semiconductor manufacture, incineration residue treatment, metal-contaminated soil improvement and so on is discharged into the environment without being treated, then contamination of the aquatic environment and soil and so on will be brought about, and there will be adverse effects on ecology and human health. In Japan, for example, standards for when discharging into public waters or sewers have thus been laid down.
Among heavy metals, for high-value precious metals such as gold (Au), platinum (Pt), palladium (Pd) and silver (Ag), recycling is economically viable, and hence these precious metals are recovered as much as possible using various methods.
On the other hand, for low-value metals such as iron and aluminum, solid material close to pure metal such as scrap is targeted for recycling, but in the case that the metal is contained in wastewater in the form of ions, recycling is not economically viable, and hence the metal is converted into solid material (sludge) by some method, and disposed of as landfill or the like.
For heavy metals that are somewhat more valuable than iron and aluminum but not as valuable as precious metals, solid metal scrap is recycled, but in the case that the metal is contained in wastewater in the form of ions, the present state of affairs is again that in most cases the metal is separated from the wastewater by some kind of chemical treatment, and is then disposed of as sludge. Examples of these heavy metals are copper (Cu), zinc (Zn), tin (Sn), nickel (Ni), lead (Pb), cadmium (Cd) and so on.
The reason that such heavy metals in wastewater are not targeted for recycling but rather are disposed of as sludge is that sludge obtained using wastewater treatment methods carried out hitherto has had a high water content, and moreover has contained large amounts of substances other than the targeted heavy metal (miscellaneous salts, compounds of other metals etc.), and hence the purity of the targeted metal has been low. To recycle a specific heavy metal from such sludge, it becomes necessary to carry out some kind of pretreatment process before the smelting process, and hence such recycling is not economically viable.
A hydroxide method and a sulfide method are known as methods for treating water distribution containing a heavy metal. Following is a comparison of the two.
(Hydroxide Method)
In the hydroxide method (alkali method) for treating wastewater containing a heavy metal, the (generally acidic) wastewater containing the heavy metal is neutralized by adding an alkaline agent such as caustic soda or slaked lime thereto, whereby the heavy metal ions and hydroxide ions react together to produce a solid having a heavy metal hydroxide as a principal component thereof, and then the solid is removed from the wastewater by being filtered off and then dehydrated.
So long as the wastewater does not contain especially toxic substances, this reaction is not a particularly difficult reaction, operation is simple, and the level of safety is high, and hence most heavy-metal-containing wastewater has been treated using this method hitherto. The reaction formula for this reaction is as in ‘reaction formula 1’ below.M+++2OH−→M(OH2).(H2O)n↓  reaction formula 1*: ‘.(H2O)n’ represents n water of crystallization molecules per molecule of the heavy metal hydroxide, and ‘↓’ represents precipitation as a solid.<Merits of Hydroxide Method>
This reaction is a simple neutralization reaction, and so long as especially toxic substances such as cyanides are not contained in the wastewater, the reaction is not difficult, operation is simple, and the level of safety is high.
The alkaline agent used is not an expensive chemical, and hence the cost of the treatment is not especially high.
However, the hydroxide method, while having merits such as the above, also has drawbacks such as the following.
<Drawbacks of Hydroxide Method>
A fundamental problem is that hydroxides M(OH2) produced through this reaction do not have sufficiently low solubility, and hence if only this treatment is used then heavy metal ions will remain in the wastewater to some extent after the treatment. There are thus cases in which the method does not conform to wastewater standards in some countries. To make up for this drawback, in general a separate additional measure such as using a chelating agent or mixing in a sulfidizing agent is often required as finishing treatment for further reducing the residual heavy metal concentration.
Regarding a plating process, to make the plating reaction proceed efficiently, a complexing agent that forms a metal complex is often used. However, in the case of heavy-metal-containing wastewater having a complexing agent mixed therein, the complexing agent has an action of impeding the production of the hydroxide, and hence the heavy metal cannot be captured sufficiently using only the hydroxide method. As a result, a large amount of heavy metal ions remains in the wastewater after the treatment, and this exacerbates the drawback described above of the solubility not being low, and hence an additional measure as described above such as using a chelating agent or mixing in a sulfidizing agent becomes yet more necessary.
The sludge generated through the hydroxide method has a high water content, this being partly due to the hydroxide having waters of crystallization as shown in reaction formula 1 above, and moreover the sludge contains large amounts of substances other than the targeted heavy metal such as salts. As a result, not only does the amount of sludge generated increase, but moreover the content of the targeted heavy metal is low, and hence the economic viability of recycling is poor, and thus, despite valuable metal being contained therein, most such sludge is disposed of as landfill or the like in actual practice.
With the hydroxide method, a plurality of heavy metals coexisting in the treated liquid all become hydroxides in a mixed state as is, and hence the individual metals cannot be separately recovered. In the case that a targeted heavy metal has other metals mixed therewith, recycling of the specific metal in the sludge thus becomes difficult.
(Sulfide Method)
In contrast with the hydroxide method described above, there is a sulfide method in which a sulfidizing agent (sodium sulfide (Na2S), sodium hydrogensulfide (NaHS) etc.) is added to heavy-metal-containing wastewater in a reaction tank, and hence a heavy metal sulfide produced through undermentioned reaction formula 2 is precipitated, and then solid-liquid separation is carried out.
The sulfide method has not been used as a major treatment method hitherto due to the drawbacks described below. The method has been used a little in a supplementary fashion as finishing treatment in the case that the residual heavy metal ion concentration has not become sufficiently low through treatment using the hydroxide method.M+++S−−→MS↓  reaction formula 2
This method, despite having drawbacks as mentioned above and as will be described in detail below, can be said to be intrinsically an excellent method in the following respects.
<Merits of the Sulfide Method>
The solubility of heavy metal sulfides is much lower than that of hydroxides, and hence the residual heavy metal concentration in the wastewater after the treatment (after solid-liquid separation) is extremely low. Additional measures for reducing the residual heavy metal ion concentration are thus unnecessary, and in principle the sulfide method is advantageous as a treatment method for sufficiently reducing the residual heavy metal ion concentration (advanced treatment).
The sulfidizing agent used in this method is sodium sulfide (Na2S), sodium hydrogensulfide (NaHS) or the like, which is inherently produced in secondary fashion in a petroleum desulfurization process; such a sulfidizing agent is more expensive than an alkaline agent, but not particularly expensive.
Due to intrinsic properties of sulfides, sludge containing a heavy metal sulfide obtained using this treatment method has a low water content, and moreover there is little contamination with impurities such as salts, and hence the heavy metal content is higher (i.e. the content of the targeted heavy metal is higher) than in the case of a hydroxide.
Moreover, heavy metal sulfides have the same components as sulfide ores, which are one type of raw material (ore) in heavy metal smelting, and hence combined with the advantages in terms of cost described above, recycling at a smelting plant is easy.
The pH region in which a heavy metal sulfide can exist as a solid without ionizing and dissolving in water varies according to the type of the heavy metal. If this characteristic is skillfully utilized, then even if a plurality of metal species are mixed together in water to be treated, depending on the types of the metals that are mixed together, it may be possible to recover each metal separately.
As the actual operation for such separate recovery, the operation of adjusting the acidity (pH) in the reaction to a range suitable for the targeted metal and then carrying out the sulfidizing reaction, precipitation and filtration is repeated, whereby a sulfide precipitate can be separately recovered for each individual metal from wastewater in which metal ions of a plurality of different types are mixed together.
If each metal can be separately recovered in this way, then this is advantageous for recycling compared with the case of a mixed metal sludge.
Despite the merits in principle of the sulfide method described above, the conventional sulfide method has the following drawbacks, and hence has not been used as a major method of treating heavy-metal-containing wastewater. As described above, the area in which this method has actually been used is as finishing treatment in the case that heavy metal ions cannot be sufficiently captured with the hydroxide method.
<Drawbacks of the Sulfide Method>
There is no suitable control method for supplying the sulfidizing agent in a suitable amount corresponding to the amount of the heavy metal ions. If the amount supplied of the sulfidizing agent is less than the suitable amount for the heavy metal, then the heavy metal ions will not be captured sufficiently as a sulfide, and hence the residual heavy metal ion concentration in the wastewater after the treatment will be high, and thus the objective of the treatment will not be achieved. Conversely, if the supply of the sulfidizing agent is excessive, then fatal problems such as generation of hydrogen sulfide gas and redissolution of the precipitate (heavy metal sulfide) or colloid formation will arise. In theory, it would be preferable to find out the heavy metal ion concentration in advance, and then add the sulfidizing agent in an amount that is a chemical equivalent thereto; however, this method is not practicable as an industrial treatment method for actual wastewater in which the amount or concentration of heavy metal ions will fluctuate. The method conventionally carried out in practice has thus been to add the sulfidizing agent in somewhat of an excess as deemed appropriate, and resolve problems that arise through the excessive addition using another method.
Hydrogen sulfide gas generated upon adding a sulfidizing agent to acidic wastewater excessively is not only malodorous, but moreover is toxic to humans, and hence if there is no method of avoiding the excessive generation of hydrogen sulfide gas, then the sulfide method cannot become a useful and powerful method for treating heavy-metal-containing wastewater.
Hydrogen sulfide gas is generated through reaction between sulfide ions S−− and hydrogen ions H+ as shown in undermentioned reaction formula 3, and is readily generated under a strongly acidic state (i.e. when the amount of hydrogen ions is high), becoming less readily generated as the state becomes alkaline, i.e. the amount of hydrogen ions becomes low.2H++S−−→H2S↑  reaction formula 3*: ‘↑’ represents discharge into the air of a gas generated in a liquid.
If the supply of the sulfidizing agent is excessive, then not only will hydrogen sulfide gas be generated, but moreover the excess sulfide ions and the heavy metal sulfide will react together to produce a polysulfide (reaction formula 4). Polysulfides have a high solubility product, and hence if this reaction occurs, then a problem of the precipitate redissolving will occur, and hence it will no longer be possible to achieve the objective of the treatment of removing the heavy metal from the wastewater. Moreover, the precipitate may form a collide, and hence coagulation ability and filter ability will become poor, and thus solid-liquid separation will become difficult.nMS+mS2−→(MnSn+m)−2m  reaction formula 4
As a method that makes up for the drawback of generation of hydrogen sulfide gas and the drawback of redissolution of the precipitate or colloid formation due to generation of a polysulfide described above (i.e. a method for coping with problems such as generation of hydrogen sulfide gas, redissolution of the precipitate, and colloid formation occurring due to excessive addition of the sulfidizing agent), adding a large amount of a compound of a polyvalent metal, i.e. a large amount of Al ions, Fe ions or the like, is carried out. As a result, a large amount of sludge compared with the original amount of the heavy metal is generated, and moreover the content of the targeted heavy metal in the sludge drops, which may result in recycling not being possible.